As an interest on an environment gradually increases in recent years, a demand has existed for the development of a small-size sensor capable of accurately obtaining different kinds of information within a short period of time. Particularly, for the purpose of making a residential space pleasant, coping with a harmful industrial environment and managing a production process of beverage and foodstuff, efforts have been made to achieve the size reduction, precision enhancement and price reduction of a micro sensor such as a gas sensor for measuring a gas concentration or the like.
The currently available gas sensor gradually evolves from a ceramic-sintered gas sensor or a thick-film-type gas sensor to a micro gas sensor having the form of a micro electro mechanical system (MEMS) due to the application of a semiconductor process technique.
From the viewpoint of a measurement method, a method of measuring a change in the electric characteristics of a sensing material of a sensor when a gas is adsorbed to the sensing material is most frequently used in the currently available gas sensor. Typically, a metal oxide such as SnO2 or the like is used as the sensing material to measure a change in the electrical conductivity depending on the concentration of a measurement target gas. This measurement method has an advantage in that it is relatively easy to use the method. A change in the measurement value becomes conspicuous when the metal oxide sensing material is heated to and operated at a high temperature. Accordingly, accurate temperature control is essential in order to rapidly and accurately measure a gas concentration. Furthermore, the gas concentration is measured after the sensing material is reset or restored to an initial state by forcibly removing gas species or moisture already adsorbed to the sensing material through high-temperature heating. Thus, the temperature characteristics in the gas sensor directly affect major measurement factors such as the sensor measurement sensitivity, the restoration time, the reaction time and the like.
Accordingly, a micro heater configured to locally and uniformly heat only the region of a sensing material is effective for efficient heating. However, if a large amount of electric power is consumed in controlling a temperature when measurement is performed by a micro gas sensor, it is necessary to use a large battery or a large power supply source although the volume of a sensor and a measurement circuit remains small. This may decide the overall size of a measurement system. Thus, in order to realize a micro gas sensor, a structure having small power consumption need to be preferentially taken into account.
Thus far, a silicon substrate having extremely large heat conductivity has been predominantly used in manufacturing most of micro gas sensors. Therefore, in order to reduce a heat loss, an etched pit or groove is formed in a sensor structure through a bulk macro-machining, thereby forming a suspended structure separated from a substrate. Thereafter, a micro heater, an insulation film and a sensing material are sequentially formed on the suspended structure. This makes it possible to partially reduce a heat transfer loss. However, this method is a manufacturing method primarily focused on wet etching that makes use of the crystal directivity of the substrate. Thus, this method has a limit in reducing the size of a sensor element. Furthermore, there is a problem in that the physical property of an etchant such as KOH (potassium hydroxide) or the like used in this method lacks compatibility with a standard CMOS semiconductor process.
FIG. 1 is a perspective view of a humidity sensor, one of micro sensors of the related art. The humidity sensor 10 includes a substrate 11, a porous anodic aluminum oxide (AAO) layer 13 formed on the substrate 11, and an electrode 15 formed on the porous anodic aluminum oxide layer 13.
The substrate 11 is made of aluminum and is formed in a substantially rectangular plate shape. The porous anodic aluminum oxide layer 13 is formed by oxidizing the substrate 11. If aluminum is oxidized, it is possible to form the porous anodic aluminum oxide layer 13 having a plurality of holes 13a formed on the surface thereof. A barrier layer is formed between the porous anodic aluminum oxide layer 13 and the substrate 11.
In this case, the holes 13a are formed to have a diameter of 60 nm or less. By forming the holes 13a to have a diameter of 60 nm or less, it is possible to prevent the holes 13a from being damaged by an etching solution. The electrode 15 is made of metal such as platinum, aluminum, copper or the like. The electrode 15 may be formed by different methods such as a vapor deposition method or the like.
The electrode 15 includes a first electrode 16 and a second electrode 17 disposed adjacent to the first electrode 16. The first electrode 16 has a plurality of electrode projections 16a protruding toward the second electrode 17. The second electrode 17 has a plurality of electrode projections 17a protruding toward the first electrode 16.
However, the electrode arrangement of the related art has a problem in that it is difficult to secure a heat generation quantity enough to heat a sensing material to a high temperature.